rtnetlink: fix rtnl_valid_stats_req() nlmsg_len check

Jakub forgot to either use nlmsg_len() or nlmsg_msg_size(),
allowing KMSAN to detect a possible uninit-value in rtnl_stats_get

BUG: KMSAN: uninit-value in rtnl_stats_get+0x6d9/0x11d0 net/core/rtnetlink.c:4997
CPU: 0 PID: 10428 Comm: syz-executor034 Not tainted 5.1.0-rc2+ #24
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x173/0x1d0 lib/dump_stack.c:113
 kmsan_report+0x131/0x2a0 mm/kmsan/kmsan.c:619
 __msan_warning+0x7a/0xf0 mm/kmsan/kmsan_instr.c:310
 rtnl_stats_get+0x6d9/0x11d0 net/core/rtnetlink.c:4997
 rtnetlink_rcv_msg+0x115b/0x1550 net/core/rtnetlink.c:5192
 netlink_rcv_skb+0x431/0x620 net/netlink/af_netlink.c:2485
 rtnetlink_rcv+0x50/0x60 net/core/rtnetlink.c:5210
 netlink_unicast_kernel net/netlink/af_netlink.c:1310 [inline]
 netlink_unicast+0xf3e/0x1020 net/netlink/af_netlink.c:1336
 netlink_sendmsg+0x127f/0x1300 net/netlink/af_netlink.c:1925
 sock_sendmsg_nosec net/socket.c:622 [inline]
 sock_sendmsg net/socket.c:632 [inline]
 ___sys_sendmsg+0xdb3/0x1220 net/socket.c:2137
 __sys_sendmsg net/socket.c:2175 [inline]
 __do_sys_sendmsg net/socket.c:2184 [inline]
 __se_sys_sendmsg+0x305/0x460 net/socket.c:2182
 __x64_sys_sendmsg+0x4a/0x70 net/socket.c:2182
 do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:291
 entry_SYSCALL_64_after_hwframe+0x63/0xe7

Fixes: 51bc860d ("rtnetlink: stats: validate attributes in get as well as dumps")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>